Thermoplastic resins such as polyethylene and polypropylene have been increasingly used for packaging materials, electric/electronic components, consumer electronic appliance/automotive components, engineering/housing-related materials, domestic miscellaneous goods, and the like. Therefore, the amount of thermoplastic resin waste has also increased. Such thermoplastic resin waste is generally incinerated or buried in a landfill site. However, incineration of thermoplastic resin waste accelerates global warming due to emission of carbon dioxide. On the other hand, thermoplastic resins are chemically stable and decomposed in the ground to only a small extent so that environmental pollution occurs.
Therefore, biodegradable polymers (e.g., polylactic acid) have been increasingly used for various applications from the viewpoint of suppressing emission of carbon dioxide and protecting the environment. Since the polylactic acid is produced using plant-derived starch without using petroleum resources, carbon is circulated through the atmosphere so that an increase in carbon dioxide concentration can be suppressed. Moreover, the polylactic acid can be decomposed in a natural environment due to microorganisms that exist in the ground and water. Therefore, the polylactic acid has attracted attention as a resin that may be effective for suppressing global warming and environmental pollution.
However, since the polylactic acid has a rigid molecular structure, the polylactic acid has inferior properties (e.g., ductility, impact resistance, and heat resistance) as compared with thermoplastic resins such as polyethylene and polypropylene. Therefore, it is necessary to improve the properties of the polylactic acid in order to utilize the polylactic acid as an alternative to thermoplastic resins such as polyethylene and polypropylene.
Attempts to improve the properties (e.g., elongation, flexibility, and impact resistance) of the polylactic acid by blending various polymers with the polylactic acid have been made. For example, a method (1) that blends a block copolymer of a low-melting-point homopolymer (melting point: 150° C. or less, e.g., polycaprolactone) and a high-melting-point homopolymer (melting point: 150° C. or more, e.g., polylactic acid) with the polylactic acid has been proposed (see Patent Document 1).
A method (2) that blends a segmented polyester, natural rubber, or a styrene-butadiene copolymer with the polylactic acid (see Patent Document 2), a method (3) that blends a thermoplastic elastomer that contains an ethylene-propylene-diene rubber with the polylactic acid (see Patent Document 3), and a method (4) that blends an epoxy group-containing olefin copolymer with the polylactic acid (see Patent Document 4), have also been proposed.    Patent Document 1: JP-A-9-137047    Patent Document 2: Japanese Patent No. 2725870    Patent Document 3: JP-A-2002-37987    Patent Document 4: JP-A-9-316310